Such a closing device is known from DE 36 44 181 C1. In a pressure cylinder, which is disposed symmetrically relative to an axis of symmetry of the mold closing device, a drive cylinder is disposed in the piston of the pressure cylinder. The pressure piston of the pressure cylinder is displaceable jointly with the movable mold carrier. Likewise, the piston rod of the drive cylinder is disposed in a stationary manner. Two compensating cylinders are disposed diametrically relative to the pressure cylinder and are in open communication with the low-pressure chamber of the pressure cylinder. During the closing movement, hydraulic medium is transferred from the compensating cylinders and from the low-pressure chamber to the high-pressure chamber, which then applies the force for keeping the mold closed. In the case of such an embodiment, however, the volume of the compensating cylinders is limited and, in order to ensure a symmetrical force introduction, an appropriate amount of precision work is necessary.
To apply greater forces, a closing device is known from DE 38 44 432 C1, wherein at least two pressure cylinders are disposed symmetrically relative to the central axis. Furthermore, the drive cylinder is disposed centrally and lies in the compensating cylinder. In practical usage with delicate injection molded parts, however, it has in fact become apparent that, despite the symmetrical disposition of the pressure cylinders, irregularities in the build-up of the pressure for keeping the mold closed may occur in certain circumstances, above all after an appropriate service-life of the machine, such irregularities affecting the precision of the injection molded parts.
In the case of a closing device known from EP 0 281 329 A2, a drive cylinder is disposed in the pressure piston of a central pressure cylinder. Volume, expelled by the pressure piston, is transferred to a compensating cylinder, whereby pressure piston and piston of the compensating cylinder are guided jointly on the movable mold carrier. However, the compensating cylinder is situated beneath the pressure cylinder and, in this respect, is disposed in an asymmetrical manner. Although the compensating cylinder is a passive cylinder, it is apparent that the asymmetrical arrangement leads to an irregular application of force with the above-mentioned disadvantages.
It is known, from EP 0 281 330 A2, to stack drive cylinder and pressure cylinder one inside the other, but to provide no compensating cylinder, though any expelled hydraulic medium is to be guided over the tank of the machine. This leads to a regular, considerable exchange of hydraulic medium, on the one hand, and this also increases the pressure build-up times, on the other hand.